1. Field of the Invention
This present invention relates to a spark plug for an internal combustion engine, the spark plug including a cylindrical metal shell, a cylindrical insulator provided in the metal shell, a center electrode provided in the insulator, and an outer electrode including an outer electrode tip welded to an outer electrode base member.
2. Description of Related Art
A spark plug having an outer electrode including an outer electrode base member and a column-shaped outer electrode tip welded to the outer electrode base member is known. This spark plug improves ignitability and durability. In the outer electrode, however, since the outer electrode tip is welded, the overall length of the outer electrode tends to become long. Therefore, the heat load at operating time increases, and the breakage strength against vibration also degrades. In order to enhance the heat dissipation property and the strength of the outer electrode, JP-B-1918156 describes a spark plug including a copper core sealed in an outer electrode, and JP-A-60-235379 describes a spark plug in which a leading end portion of a metal shell is extended longer toward a leading end side or a cross-sectional area of an outer electrode is increased.
For low fuel consumption and low emission, high ignition performance and also high output performance are demanded for a recent internal combustion engine. Accordingly, an internal combustion engine with a high compression ratio has been developed, but the heat amount received by the spark plugs thereof to further increase. Since the size of an outer electrode also needs to be reduced because of a requirement for a smaller diameter of a spark plug, the heat resistance and breakage resistance properties of the outer electrode become more and more strict. To solve this problem, shortening the length of the outer electrode is the most effective. However, it is known that a spark plug of a multiple electrode type and a spark plug of a semi-surface type, which can shorten an outer electrode, are inferior in ignitability to a spark plug of a parallel electrode type requiring a relatively long outer electrode.